1. Field of the Invention
The present invention relates to a wide-angle catoptric system, which is used in cameras having wide-angled fields of view for observations, inspections and so forth, and which forms images of objects.
2. Description of the Related Art
The function of optical systems for use in cameras is to bend light by the refractive action of light and to produce images of an object on detectors. Optical elements having such refractive action include, for instance, lenses bending light by using the difference in refractive indexes, reflecting mirrors bending light by using reflection, and so forth.
As light transmits through lenses, lenses should be made of a material having sufficient transmittance for a desirable wavelength band. For particular wavelength bands such as ultraviolet rays and infrared rays, the materials of lenses are limited to expensive types. Moreover, the refractive index of lens materials usually has chromatic aberration that is different in size depending on the wavelength of light, so that a complex correction, the so-called achromatism, has to be carried out by combining two or more lenses having different refractive index changes in respect to a wavelength, or the like, in order to provide consistent imaging properties over a broad wavelength band.
There is no limitation on the materials for reflecting mirrors as long as their reflecting surfaces can be coated with a reflecting material having sufficient properties, so that economical optical systems may be obtained for any wavelength band. Additionally, since reflection does not rely on the wavelength of light, reflecting mirrors have no chromatic aberration and an optical system, having consistent imaging properties over a broad wavelength band, may be easily obtained.
However, in a catoptric system, incident beams to a reflecting mirror and the reflected beams appear on the same side relative to the reflecting mirror, so that the following reflecting mirror will be located on the same side as the incident beams thereof. Accordingly, the phenomenon whereby the following reflecting mirror intercepts the incident beams, termed shading, is likely to occur. A bright optical system cannot be obtained as the quantity of incident light decreases due to shading.
In order to avoid shading, there is, for example, a method whereby incident beams are tilted relative to an optical axis so as to provide an angle between the input direction and the output direction of beams. The following reflecting mirror is usually deposited in the output direction of beams, so that the following reflecting mirror will not overlap the incident beams if the input direction and the output direction of beams are different with an angle therebetween, thus not generating shading.
However, optical systems are generally designed on the basis of paraxial imaging that is achieved when incident beams are parallel to an axis of the optical systems. Accordingly, paraxial imaging will be out of focus if incident beams are tilted relative to an optical axis, thus intensifying aberration. Images will be blurred and picture quality will deteriorate, as aberration increases.
FIG. 5 is a conventional wide-angle catoptric system disclosed in U.S. Pat. No. 4,598,981: “WIDE-ANGLE FLAT FIELD TELESCOPE”. The system has a brightness of F/4 and a field of view of 30° times 20°. In the optical system of FIG. 5, a primary reflecting mirror 1 has a convex spherical surface. A secondary reflecting mirror 2 has a concave ellipsoidal surface. A tertiary reflecting mirror 3 has a concave spherical surface. A circular diaphragm 4 is placed in close proximity to the primary reflecting mirror 1.
Additionally, a luminous flux 5 is the flux of incident beams to the primary reflecting mirror 1, and a luminous flux 6 is the flux of output beams from the secondary reflecting mirror 2. An optical axis 7 is a straight line connecting the curvature center of the primary reflecting mirror 1 to the curvature center of the secondary reflecting mirror 2. The center of curvature of the tertiary reflecting mirror 3 and the center of the diaphragm are also on the axis 7, providing a co-axial configuration. An incident beam is relayed onto image plane 8. Tilting incident beams relative to the optical axis 7 prevents the shading caused by overlapping beams.
In catoptric systems, the major aberrations deteriorating the picture quality of images are spherical aberration, coma, field curvature, and astigmatism. By setting parameters such as the radii of curvatures of reflecting mirrors and surface spacing based on the following principles, the wide-angle catoptric system mentioned above reduces each aberration.
Spherical aberration is caused by the deviation of focal positions between central beams and peripheral beams. In the wide-angle catoptric system, optical parameters are determined to provide almost the same deviation of a focal position of peripheral beams by the secondary reflecting mirror 2 as the deviation of a focal position of peripheral beams by the tertiary reflecting mirror 3 in the reverse direction, so that the deviations cancel each other, reducing spherical aberration. Accordingly, the spherical aberration by the primary reflecting mirror 1 still remains.
Coma is caused by the deviation of focal positions between central beams and peripheral beams for the light input with an angle relative to the optical axis 7. The above-mentioned wide-angle catoptric system determines optical parameters so as to mutually cancel the deviations at all three mirrors, the primary reflecting mirror 1, the secondary reflecting mirror 2 and the tertiary reflecting mirror 3, thus reducing coma.
Field curvature is the phenomenon whereby an image plane is curved with a curvature, and the radius of curvature thereof is expressed in a so-called Petzval sum. In the above-mentioned wide-angle catoptric system, the radii of curvatures of the primary reflecting mirror 1, the secondary reflecting mirror 2 and the tertiary reflecting mirror 3 are set so as to provide a zero Petzval sum, thus providing a zero curvature of the image plane and eliminating a field curvature.
Astigmatism is found for the light, which is input with an angle relative to the optical axis 7, when reflecting mirrors have different shapes between the tangential direction and the sagittal direction, and appears as the difference in curvature radii of an image plane between the tangential direction and the sagittal direction. In the wide-angle catoptric system described above, optical parameters are determined to provide almost the same difference in curvature radii of the image plane at the primary reflecting mirror 1 as the difference in curvature radii of the image plane at the tertiary reflecting mirror 3 in the reverse direction, so that the differences cancel each other, reducing astigmatism. Thus, the astigmatism by the secondary reflecting mirror 2 still exists.
In order to obtain a bright optical system having a small F-number in the wide-angle catoptric system as mentioned above, the aperture of the diaphragm 4 should be widened so as to increase the quantity of incident light. However, if a luminous flux is broadened by enlarging the aperture of the diaphragm 4, shading will occur. Particularly, the luminous flux 5 of incident beams to the primary reflecting mirror 1 and the luminous flux 6 of output beams from the secondary reflecting mirror 2 are in close proximity. Therefore, as the two luminous fluxes overlap each other by broadening the luminous flux, shading will occur at the overlapping section thereof.
In order to avoid shading, it is necessary to provide a large angle between incident beams and an optical axis. However, if the angle is large between incident beams and the optical axis 7, aberrations will also become serious and the picture quality will deteriorate sharply. Therefore, a problem existed in that conventional wide-angle catoptric systems could not provide a bright optical system.